Someone who is dangerously skinny and skeletal-looking can be described as emaciated. It's probably how you'd start to look after a few weeks in the wilderness with only berries and bugs for dinner.
The adjective emaciated evolved from the Latin emaciatus, meaning to “make lean, waste away.” An emaciated person or animal isn't just thin. They're bony, gaunt, and most likely undernourished, often from illness. So if an emaciated stray cat shows up on your doorstep, give it a bowl of milk and maybe pay a visit to the vet.